My opinion, It's pretty simple, if a user cant keep himself secure on windows the last thing he needs to do is jump on a server environment. No matter what windows manager you pretty it up with Linux is not a desktop OS.
I will say I havn't played with any of the 2009 round of "glamour linux" to see how secure there default installs are, but when I last played with them there were always default services enabled(ssh,ftp,http,cups), there have been a number of exploits for these services and mass scanners to exploit them. Even if they were disabled, there's still browser exploits which is the main problem. If a regular user doesn't keep there windows computer up to date they wont keep there linux computer up to date.
Is there less of a chance of them being infected on Linux than on windows? absolutely. But if you get exploited on linux, you're system is basically done and needs to be wiped because you dont know if or what your kernels been recompiled with. While in my experience, on windows, 99% of the time you can clean your computer.
Then there's the point of, if your 72 year old grandmother just needs to get on yahoo mail then sure linux is the right way to go, because you can secure it for her. I just believe telling a windows user who has had no experience with anything other than windows to install linux, is a terrible idea.